Liquid adhesives, such as temperature and/or pressure sensitive adhesives, are frequently dispensed as a continuous adhesive filament with a controlled pattern onto a narrow substrate or a narrow width of a larger substrate. Conventional patterns have been created by impacting the adhesive filament with a plurality of air jets as the filament exits the discharge outlet of the dispenser or applicator. In the hot melt adhesive dispensing industry, one dispensing technique of this type is generally known as controlled fiberization or CF™, and is described, for example, in U.S. Pat. No. 4,785,996. The air jets impart a swirling effect to the adhesive filament that produces a generally back-and-forth pattern, which may have a regular or irregular appearance. Other conventional adhesive filament dispensing techniques and apparatus have been used for producing back-and-forth patterns of adhesive on a substrate, such as the vacillating pattern disclosed in U.S. Pat. No. 6,077,375 and the omega-shaped pattern disclosed in U.S. Pat. Nos. 6,461,430, 6,200,635 and 6,197,406.
Controlled fiberization and like filament dispensing techniques are used in the manufacture of hygienic articles, such as diapers, incontinence pads and other absorbent undergarments. In particular, controlled fiberization is a popular manufacturing technique employed for elasticizing specific areas of hygienic articles, such as the waistbands, leg cuffs, and standing leg gathers of diapers and adult incontinence products. To that end, continuous adhesive filaments are dispensed onto one or more individual moving elastic strands, either before or after the strand has contacted a substrate, for bonding each strand to the substrate. The adhesive filament drapes lengthwise along the moving strand about its circumference and secures the strand to the substrate after contact. In this manner, overlapping portions of the same material may be bonded together with each stretched elastic strand secured therebetween or two distinctly different substrates may be bonded together as a laminate with the stretched elastic strand secured therebetween.
With reference to FIG. 1, a conventional method of dispensing an adhesive filament 12 onto a moving strand 14 is illustrated. The strand 14 moves in a machine direction 19 at speeds of up to 1200 feet per minute past an adhesive applicator 16. The adhesive filament 12 is dispensed by the adhesive applicator 16 onto the strand 14 in a generally back-and-forth pattern relative to the direction of motion of strand 14 in the machine direction 19. The back-and-forth pattern is produced by multiple air jets 17 that steer the adhesive filament 12 transversely relative to the travel direction of strand 14. The transverse movement of the filament causes certain points or sections 12a of the filament 12 to contact the top of the strand 14 and other filament sections 12b to be airborne. The air jets 17 impart angular momentum to the airborne sections 12b that cause them to wrap about the circumference of the strand 14 downstream of the adhesive dispenser 16 until at least most of the adhesive filament 12 contacts the strand 14. The strand 14 is contacted with a substrate 18 and will be adhesively bonded to the substrate 18 by the adhesive filament 12. The strand 14, if elastic, may be stretched so that, upon attachment to the substrate 18, the substrate will be elasticized generally along a line defined by strand 14.
A significant problem is routinely encountered in the dispensing of such adhesive filaments 12 onto a moving strand 14. Specifically, the rapid movement of the strand 14 in the surrounding static or stagnant air induces air resistance or aerodynamic drag on the airborne sections 12b of filament 12. The direction of the force applied by the drag on the airborne filament sections 12b is opposite to the machine direction 19. As a result, the airborne filament sections 12b have a velocity that is different than the velocity of the strand 14 in the machine direction 19. The effects of drag persist until such time that the airborne filament sections 12b are wrapped about and adhesively bonded to strand 14. The drag causes the airborne filament sections 12b to stretch and lengthen relative to the contacting sections 12a. The lengthening induced by drag increases with increasing linear velocity of strand 14 in the machine direction 19. The pattern of the adhesive filament 12 on the strand 14 becomes significantly irregular so that lengths of the strand 14 may not be adequately coated and other portions may be heavily coated. As a result, the adhesive-coated strand 14 is not uniformly bonded along its length after it is applied to substrate 18. This adversely affects the properties of the bonded elastic strand 14 and substrate 18, such as product flexibility and softness.
Another problem occurs in those applications in which multiple closely-spaced moving strands are each receiving a discrete adhesive filament. Specifically, stretching or lengthening due to the drag forces can cause an adhesive filament intended to be received on one moving strand to contact and be received instead on an adjacent moving strand. A similar problem occurs in applications in which a single filament is being intentionally applied to multiple strands as certain strands may receive adhesive or a relatively heavy coat of adhesive while other strands are uncoated or irregularly coated. Either situation results in improper adhesive application and may result in a loss of usable product yield due to unadhered strands.
For these and other reasons, it would be desirable to provide an apparatus and method to compensate for the effects of the ambient environment on an adhesive filament being applied to a moving narrow substrate.